Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, in order to supply a fuel gas such as a hydrogen-containing gas and an oxygen-containing gas such as the air to the anode and the cathode of the electrolyte electrode assembly, respectively, a fuel gas channel and an oxygen-containing gas channel are formed along surfaces of the separators.
For example, in a flat stack fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-120589, as shown in FIG. 22, a separator 1 stacked on a power generation cell is provided. The separator 1 is formed by connecting left and right manifold parts 2a and a part 2b at the center where the power generation cell is provided, by joint parts 2c. The joint parts 2c have flexibility.
The manifold parts 2a have gas holes 3, 4. One gas hole 3 is connected a fuel gas channel 3a, and the other gas hole 4 is connected to an oxygen-containing gas channel 4a. The fuel gas channel 3a and the oxygen-containing gas channel 4a extend in a spiral pattern into the part 2b, and are opened to a fuel electrode current collector and an air electrode current collector, respectively, at positions near the center of the part 2b. 
In the above conventional technique, the part 2b for placing the power generation cell is provided at the center of the separator 1. In the structure, it is not possible to suitably heat the fuel gas and the oxygen-containing gas flowing through the gas holes 3, 4 by the heat produced in the power generation of the fuel cell. Therefore, it is not possible to improve the heat efficiency or facilitate thermally self-sustained operation.
Further, one power generation cell is provided in each separator 1. Therefore, in order to obtain the high output, it is necessary to stack a large number of power generation cells, and the overall size of the fuel cell in the stacking direction becomes considerably large. Further, if any power generation failure occurs in one of the power generation cells, the failure causes malfunction in power generation of the entire fuel cell. Therefore, power generation cannot be performed efficiently.